Barcarolle by F.Schubert for Flute and piano duet, 10 pages (7 pages flute and piano score, 3 pages flute solo) 2:25 min, with downloadable mp3 just for audio help.

Ludwig van Beethoven's Piano Sonata No. 8 in C minor, Op. 13, commonly known as Sonata Pathétique, was written in 1798 when the composer was 27 years old, and was published in 1799. It has remained one of his most celebrated compositions.
Here is an arrangement for Cello and Piano accompaniment for Adagio Cantabille (op.13-II).
With downloadable mp3 for audio help.  Eight pages (6 page Cello and Piano and 2 pages Cello), 3:56' min.

La Marseillaise was written in 1792 by Claude Joseph Rouget de Lisle in Strasbourg after the declaration of war by France against Austria, and was originally titled "Chant de guerre pour l'Armée du Rhin". The Marseillaise was a revolutionary song, an anthem to freedom, a patriotic call to mobilize all the citizens and an exhortation to fight against the tyranny and the foreign invasion. The French National Convention adopted it as the Republic's anthem in 1795.
Here is a piano duet 4 hands arrangement, 4 pages (2 pages both hands, 1 page prima, 1 page seconda) with downloadable mp3s just for audio help.

12 short duets for 1 piano 4 hands.
Includes Reverie, A little waltz, Merry go round, March, Rememberance, Waltz, Quite contented, Bag-pipes, Bolero, The Brook, Lullaby, Good-by.
Easy piano,15 pages.
With downloadable mp3s for audio help.

A romantic guitar and piano duet (19th century).  
Early intermediate piano, easy to early intermediate guitar, with downloadable mp3 for audio help.
5 pages (3 pages both guitar and piano, 2 pages guitar with tablature), 3' min.